<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<input type="text" placeholder=半径 id="a">
<input type="text" placeholder=周长 id="b">
<input type="text" placeholder=面积 id="c">


<script>

    var p = Math.PI;

    var t = prompt("输入圆的半径");
    t = Number(t);

    //圆的周长 = 直径 * π = 半径 * 半径 *π
    var zc = t * 2 * p;
    //圆的面积 = 半径的平方 * π = 半径 *半径 *π
    var mj = t * t * p;

    document.querySelector("#a").value = t;
    document.querySelector("#b").value = zc;
    document.querySelector("#c").value = mj;


</script>

</body>
</html>